The reason that oral sex is so much safer than getting fucked is that the throat is not as vulnerable to infection as the arse is. Saliva has properties that can disable some infections, including HIV, and so there needs to be quite a lot of HIV present for infection to take place.Also, the mouth and throat are not as efficient at absorbing liquids into the bloodstream as the lining of the arse is. Even if you swallow cum, any HIV that may be there will usually be killed by the strong acids in your stomach.